There are many existing processes for forming thermoplastic (thermoforming) articles including vacuum molding, injection molding, extrusion molding and the like. A perennial problem with many existing thermoforming processes, particularly, when dealing with large thermoformed materials and continuous fiber reinforcements, is that voids appear between the thermoformed material and the mold. Another problem that occurs when there are recessed areas in the mold is resin pooling in the recessed areas when dealing with pre-consolidated thermo-compositess. A further problem is that folds and pleats in the thermoformed product occur at the corners and other areas of the thermoformed product when complex and deep concave or convex articles are formed.
Fiber composite materials that require heating and then a molding force in the form of either vacuum or positive pressure are commonly referred to as pre-consolidated thermoformed fiber composites. Most of the pre-impregnated materials used in conjunction with fabricating techniques using an autoclave are generally classified as thermoformed fiber composite materials. Products made with pre-impregnated materials using autoclave technology are very strong and lightweight. However, the materials, tooling and equipment required to make parts using pre-impregnated materials are very expensive. The heating, forming and cooling cycle times are very long as well.
Twintex™ is a drape molded thermoformable product manufactured by St. Gobain-Vetrotex Corp. The product is fully described in a manual entitled “Twintex Vacuum Moulding Manual”, the contents of which are incorporated herein by reference. It is in the form of a commingled glass fibre-polypropylene fibre composite woven cloth that is very similar in appearance to the conventional fiberglass woven cloth that is used in combination with liquid resins (polyesters, acrylics or epoxies) to fabricate a wide variety of products such as boats, automotive body panels and housing construction materials. The material is drape molded on the mold prior to heating and then consolidated using vacuum or pressure. However, unlike conventional fiberglass products that require the saturation of the fibers with a liquid resin such as a polyester, an acrylic or a two part epoxy (commonly referred to as thermo-set fiber composites), the Twintex material requires heating to 400 degrees Fahrenheit and then either pressure or vacuum forming the part for the duration of the cooling cycle. The difference between Twintex and a pre-impregnated composite is that the glass fibers in Twintex are “commingled” with heat formable polypropylene fibers, whereas the fibers in a pre-impregnated material (typically Kevlar™ or carbon graphite) are either “pre-coated” or pre-formed in a flat sheet with a heat-formable version of an epoxy or an acrylic resin. The Twintex material is typically much easier to handle and apply, has much shorter heating and molding cycle times, requires less pressure for molding and costs far less per pound than pre-impregnated materials.
The Twintex material on a strength to weight ratio, and on a price per pound ratio outperforms products consisting of far more expensive thermo-set material combinations such as Kevlar/epoxy resin laminations in the areas of strength, impact resistance and abrasion resistance. The cycle times for heating, pressure forming and cooling parts made of the Twintex material or other thermoformable material can also be substantially shorter than the cycle times required to saturate and cure fiberglass parts made using liquid resins.
There are many methods of producing tooling and parts made with Twintex material. However, all of the methods involve either expensive tooling and equipment, long heating and cooling cycle times, or a combination of the two.
The inventor herein is aware of the following patents which may be considered relevant to the subject invention.                US 2002/0,012,765 A1        U.S. Pat. No. 3,949,125        U.S. Pat. No. 3,414,456        U.S. Pat. No. 5,655,861        U.S. Pat. No. 5,529,826        U.S. Pat. No. 4,267,142        U.S. Pat. No. 4,576,776        U.S. Pat. No. 4,716,067        U.S. Pat. No. 5,217,555        U.S. Pat. No. 5,725,704        U.S. Pat. No. 5,928,597        